This invention relates generally to liquid filtration systems and, more specifically, to traveling bridge-type cleaning devices for sequentially cleaning a plurality of downward flow, suspended solids filter units located within a relatively large filtration tank.
It is well known in the art to utilize filtration tanks divided into a plurality of filter cells, one adjacent the other, and all containing a multi-layer arrangement of granular filter media such as sand, gravel and the like. In downward flow filtration systems of this type, water or other liquid containing suspended solid particles is introduced into the filtration tank from above and clarified water is drawn off from a chamber beneath the filter cells. During downward flow through the individual cells, particulate matter is entrapped within the layers of granular filter media. Eventually, the particulate matter clogs the filter media, thereby reducing the filtering capability of the system. Thus, there is a need for periodic cleaning of the filter cells. It is also known to maintain such units in continuous operation during cleaning by the use of traveling bridge devices which move from one filter cell to the next, to backwash individual cells while permitting the filtration process to continue in the remaining cells.
Examples of traveling bridge apparatus of this type may be seen in U.S. Pat. Nos. 4,540,487; 4,486,307; 4,133,766; 3,984,326; 2,235,227, and 649,409. Typically, traveling bridge systems include an overhead carriage, movable along tracks, guideways or the like, which carries, e.g., a backwash hood successively engageable with the upper end of each filter cell. For a downflow type filter, water or other treatment liquid is generally introduced into the cell from below, in a counterflow arrangement to the normal filtering direction. The backwash hood typically includes a suction head for drawing out fluid and debris forced to the surface of the filter cell as a result of the backwash. As the backwash of an individual cell is completed, the traveling bridge will index the backwash hood to the next adjacent cell. See, for example, U.S. Pat. No. 4,308,141, in which a modular filter system is disclosed, and wherein a traveling bridge type backwash system is indexed to successive filter units.
There are also known in the prior art devices for breaking up a mat or layer of material which forms on the surface of a filter bed. For example, in U.S. Pat. No. 2,335,749, a pair of surface raking mechanisms are utilized on either side of a backwash caisson. In U.S. Pat. No. 2,194,071, a overhead surface wash system is disclosed wherein a spray head having fixed jets is suspended from a carriage, enabling wash liquid to be directed downwardly onto the upper surface of the filter bed as the carriage moves along the length of the tank. A conventional, stationary backwash system is utilized to wash the loosened material upwardly into one or more stationary discharge troughs.
U.S. Pat. Nos. 4,152,265 and 4,454,034 disclose traveling bridge filtration systems which combine backwash operations with the utilization of mechanical scarifying blades which are said to break up and loosen the mat on the surface of the bed.
In the present invention, a unique combination of filter cell cleaning devices is provided in a traveling bridge configuration. In one exemplary embodiment, an elongated, rectangular filtration tank is divided into a plurality of individual cells by a plurality of laterally spaced, substantially parallel partitions extending across the width of the tank. The tank is further provided with tracks or guideways supporting a traveling bridge for movement along the top of the tank, in a direction transverse to the orientation of the upright partitions. In an exemplary embodiment of the invention, the traveling bridge comprises a carriage from which is suspended surface wash means as well as backwash means, located within a single hood assembly, arranged for sequential sealing engagement with each filter cell.
The single surface wash/backwash hood assembly of this exemplary embodiment of the invention extends across substantially the full width of the tank, i.e., along substantially the entire length of the individual filter cells. The hood per se includes a pair of lower side wall portions which are spaced apart a distance corresponding to the distance between the individual cell partitions, as well as upper side wall portions which are inclined toward each other and which intersect at a hood apex.
Located within the hood, and extending substantially parallel to, and midway between, the lateral lower side wall portions of the hood, is a surface wash manifold, extending substantially across the width of the tank, and provided at spaced positions along its length with a plurality of discharge nozzles from which liquid is dispensed under pressure onto the upper surface of the filter bed. The surface wash manifold is connected to a substantially vertically oriented surface wash conduit which extends upwardly through the hood to a first fluid surface wash pump mounted on the traveling bridge carriage.
In the exemplary embodiment, water is supplied to the pump from a clean water trough adjacent the tank, although it will be appreciated that water may also be supplied from any fresh or clean water source.
A horizontal backwash manifold is also located within the hood, above the surface wash manifold, and generally coextensive therewith. The backwash manifold is preferably located in the area of the inclined upper wall portions of the hood. The backwash manifold is connected substantially midway along its length to a vertical suction conduit which also extends upwardly, through the hood, to a second fluid backwash pump mounted on the travelling bridge carriage. This second pump is a suction pump, the inlet side of which is connected to the vertical suction conduit.
The horizontal backwash manifold is provided with a plurality of uniformly arranged apertures extending along its length and about its periphery to uniformly draw liquid and accompanying debris upwardly out of a filter cell in a direction opposite the normal filter flow direction to thoroughly backwash the filter media.
In order to index the hood assembly to successive cells along the length of the filtration tank, the hood assembly is reciprocable vertically a distance sufficient to enable the hood assembly to disengage from the cell partitions or walls, and to clear the adjacent cell partitions during lateral movement to the next cell. To this end, portions of the vertical surface wash conduit and vertical backwash conduit, respectively, may be provided with flexible, e.g., bellows-type, connectors which permit a relatively small degree of vertical movement of the hood structure vis-a-vis the vertically fixed carriage.
Accordingly, in its broadest aspect, the present invention relates to a traveling bridge filter cleaning system including a liquid filtration tank divided into a plurality of adjacent filter cells, each of the cells containing filter media; a traveling bridge carriage located above the tank and movable along the tank, the carriage mounting surface wash means and backwash means within a single hood assembly adapted to sealingly engage successive ones of the filter cells.
For each individual cell, the carriage mounted first and second pumps may be simultaneously activated so that the mat or crust of material present on the upper surface of the filter bed is broken up by the surface wash, while at the same time, the backwash expands and cleans the filter media from below, drawing dirt and debris as well as the broken surface crust or mat material upwardly into the suction conduit located in the hood assembly. In this way, the filter media is effectively cleaned, the formation of mudballs is prevented, and the full filtration capability of the cell is restored.
It will therefore be appreciated that the present invention provides a unique combination of surface wash and backwash means in a single hood assembly within a traveling bridge filter cleaning system, which effectively cleans successive cells of a multi-cell tank without disturbing the filtering process in the remaining cells.
Additional objects and advantages of the subject invention will become apparent from the detailed description which follows.